The present invention relates to devices and methods for collecting and redistributing liquid descending in packed exchange columns for heat and/or mass transfer processes. The invention has particular application in cryogenic air separation processes utilizing distillation, although it also may be used in other heat and/or mass transfer processes which use packing (e.g., random or structured packing). The present invention also relates to methods for assembling devices for collecting and redistributing liquid descending in packed exchange columns.
As used herein, the term “column” (or “exchange column”) means a distillation or fractionation column or zone, i.e., a column or zone where liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, such as by contacting of the vapor and liquid phases on packing elements or on a series of vertically-spaced trays or plates mounted within the column.
The term “packing” means solid or hollow bodies of predetermined size, shape, and configuration used as column internals to provide surface for the liquid to allow mass transfer at the liquid-vapor interface during countercurrent flow of two phases. Two broad classes of packings are “random” and “structured.”
“Random packing” means packing wherein individual members do not have any particular orientation relative to each other or to the column axis. Random packings are small, hollow structures with large surface area per unit volume that are loaded at random into a column.
“Structured packing” means packing wherein individual members have specific orientation relative to each other and to the column axis. Structured packings usually are made of thin metal foil, expanded metal, or woven wire screen stacked in layers or as spiral bindings; however, other materials of construction, such as plain sheet metal, may be used.
In processes such as distillation or direct contact cooling, it is advantageous to use structured packing to promote heat and mass transfer between counter-flowing liquid and vapor streams. Structured packing, when compared with random packing or trays, offers the benefits of higher efficiency for heat and mass transfer with lower pressure drop. Structured packing also has more predictable performance than random packing.
Cryogenic separation of air is carried out by passing liquid and vapor in countercurrent contact through a distillation column. A vapor phase of the mixture ascends with an ever increasing concentration of the more volatile components (e.g., nitrogen) while a liquid phase of the mixture descends with an ever increasing concentration of the less volatile components (e.g., oxygen).
Various packings or trays may be used to bring the liquid and gaseous phases of the mixture into contact to accomplish mass transfer between the phases. The use of packing for distillation is standard practice and has many advantages where pressure drop is important.
Initial presentation of liquid and vapor to the packing is usually made by means of distributors. A liquid distributor, the role of which is to irrigate the packing substantially uniformly with liquid, is located above the packing, while a vapor distributor, the role of which is to create substantially uniform vapor flow below the packing, is located below the packing. In addition to the vapor distributor, a liquid collector is also located below the packing, the role of which is to collect all the liquid leaving the packing and direct it further down the column. It is common for the liquid collector and the vapor distributor to be encompassed in the same device, which performs both roles.
U.S. Pat. No. 4,744,929 (Robinson, et al.) entitled “Support Device for a Packed Column” discloses a support device for a packed column. The support device is intended to function both as a support device for the packing material and as a collector for descending liquid.
The prior art includes many other devices for collecting and redistributing liquid in packed columns. Generally, these devices include independent pieces of equipment which need to be supported at specific locations in the column and which take up space (height) within the column, making the column taller. This results in increased costs for fabrication, transportation, installation, and operation of the column.
It is desired to have an apparatus and a method for collecting and redistributing a flow of a liquid descending in an exchange column which require relatively little additional column height and associated costs.
It is further desired to have an apparatus and method for collecting and redistributing a flow of a liquid descending in an exchange column in which the liquid collector is easily integrated with the support grates or other members.
It is still further desired to have a new, more efficient apparatus and method for collecting and redistributing a flow of liquid descending in an exchange column.
It is still further desired to have an apparatus and a method which effectively distribute vapor while collecting and redistributing a flow of liquid descending in an exchange column.
It is still further desired to have an apparatus and a method for collecting and redistributing a flow of liquid descending in an exchange column which overcome the difficulties, problems, limitations, disadvantages, and deficiencies of the prior art to provide better and more advantageous results.
It is also desired to have a method for assembling an apparatus for collecting and redistributing a flow of a liquid descending in an exchange column which affords better liquid distribution than the prior art, and which also overcomes many of the difficulties and disadvantages of the prior art to provide better and more advantageous results.